The birds' vibrant plumage appears to act like a flashing billboard, broadcasting the males' reproductive superiority to females eager to produce offspring.

The advertisement likely finds an appreciative audience in female great tits, since snagging a male with high-quality sperm isn't exactly a lark.

That's in part because free radicals threaten sperm cells in many animals, including humans. Created by cells when stressed by pollution and other factors, free radicals are groups of oxygen-activated atoms that can damage sperm cells, weakening their swimming ability. (Learn how DNA works.)

Many animals' bodies produce antioxidants that fight free radicals—including male great tits. The birds have an antioxidant called carotenoid that not only defends against free radicals but also gives their breast feathers a yellow hue.

The new research shows that in males with more carotenoid, and thus more color, the sperm is better able to withstand a free radical onslaught—and females seem to know it.

Though human males don't display ornate natural plumage, the bird research has some parallels to people, said study leader Fabrice Helfenstein, an evolutionary ecologist at the University of Bern in Switzerland.

For instance, free radicals are thought to be a major cause of infertility in people.

Likewise, some studies have shown that men with more attractive faces have better quality semen.

Male Massage

To investigate whether more colorful great tits have more resilient sperm, Helfenstein and colleagues increased the parenting "workload" of 29 nesting pairs in Bern, Switzerland, in 2008. Such added stress is known to boost free radicals.

Two days after a great tit mother and father—which share parenting duties—hatched their brood, the team added an additional two baby birds per nest.

The scientists also observed a control group of 31 nests that were left undisturbed.

At 7 and 15 days after the broods hatched, the scientists trapped both the stressed and nonstressed males and massaged their cloacas—all-purpose openings found in many animals—to make the males ejaculate.

Under the microscope, the scientists observed how fast and robustly the birds' sperm moved. Unlike human sperm, which make fanciful loops and twirls, bird sperm swim "very straight," Helfenstein added as an aside.

The results showed that among nonstressed males, sperm robustness was about equal.

Among the stressed males, though, great tits with paler plumage had more listless sperm than colorful males—suggesting that having more carotenoid helps cope with stress.

But in samples taken on day 15, the paler birds' sperm quality was only slightly reduced by stress. This means that even pale great tits can eventually compensate when times get tough, according to the recent study in the journal Ecology Letters.

In a further step, the team also force-fed males on two separate occasions with carotenoid-laced maggots. The control group was given placebo maggots.